Microvascular endothelial (MVE) cell preparations from rat and guinea pig fat ped, heart and brain contained both the M and P-type (5 hydroxytryptamine, i.e. 5-HT and Beta-naphthol) sulfating enzyme phenolsulfo- transferase (PST). A species as well as tissue difference was found in the types and level of PST in the various MVE cell preparations, reflecting the heterogenicity of the endothelium in regard to PST. PST was found to metabolize 3H5-HT to 3H5-HT-SO4 in rat MVE cells. The inhibition of 3H5-HT retention by reserpine and the increase in levels of 3H-5-hydroxyindole acetic acid (3H5-HIAA) suggested a membrane enclosed storage site for 5-HT. However, reserpine did not affect the level of 3H5-HT-SO4 formed, which displays the complex nature of the mechanism responsible for amine storage and metabolism in MVE cells.